To record a color image on resin packing material or the like, image recording is mainly performed by gravure printing in which ink is put into recessed portions formed on a printing surface and directly transferred to a recording medium. Gravure printing has features that allow it to colorfully express characters, symbols, pictures, for example, and perform fast recording on recording media in a large amount. Herein, for example, in case of recording an image on a transparent packing material, recording is sometimes performed on a back side of the packing material to maintain durability against rubbing of the recording surface, which is called ‘back-printing’. On the other hand, in case of recording an image on an opaque packing material, the recording is performed on the front side of the packing material, which is called ‘front-printing’.
In case of recording characters, symbols, or pictures on a packing material, with a background color in a special color such as white, gravure printing is a useful recording technology having the above features. However, in gravure printing, it is necessary to prepare a plate in a pre-process prior to actual image recording, and an extra cost and time are consumed to prepare this plate.
In this situation, attention is paid to recording technologies which use an inkjet recording apparatus without requiring a pre-process. As an inkjet recording apparatus capable of recording characters, symbols, or pictures with process-color inks together with a background color in a special color such as white on a transparent or translucent resin packing material, there have been developed inkjet recording apparatuses having a control device to make it possible to selectively perform front-printing and back-printing (as an example, see Patent Document 1).
Further, as inkjet recording apparatuses applicable to various recording media, inkjet recording apparatuses which use photo-curable ink are known. In such an inkjet recording apparatus, light is emitted to ink having landed on a recording medium, and thus, the ink is instantly cured, by which the ink sinks or bleeds into the recording medium little. Therefore, it is possible to perform image recording, not only on a plain paper sheet, but also on a recording medium having no image receiving layer or ink absorbance, such as a plastic or metal recording medium.
In general, in case of performing image recording by the use of a recording medium having an image receiving layer, most ink is absorbed by the recording medium. However, in case of performing image recording by the use of photo-curable ink and a recording medium having no image receiving layer, ink is cured and fixed simultaneously as light is irradiated to the ink, without being absorbed by the recording medium. Therefore, the ink remains on the surface of the recording medium in a state that the portions where ink landed are raised with cured ink.
As in this case where cured layers without flatness are present on a recording medium surface, namely in an image, uneven gloss is visually recognized, and thus image quality as a whole image is degraded, causing a problem that precise image recording can not be achieved.
In this situation, in order to reduce the difference in feeling of gloss between image portions and non-image portions, inkjet printing apparatuses have been developed which properly adjust the composition of a transparent ink which is not colored at all or virtually not colored, and jet this transparent ink onto non-image portions on a recording medium to smooth the surface of the recording medium (for example, see Patent Document 2).
[Patent Document 1] TOKKAI No. 2003-285427
[Patent Document 2] TOKKAI No. 2003-191601
However, in case of performing front-side printing by an inkjet recording apparatus using UV curable ink, process-color ink is jetted after special color ink such as white ink as the background color is jetted onto a packing material of a transparent or opaque resin. Herein, ink is cured and fixed simultaneously as light is irradiated to the ink and is not absorbed by the recording medium. Thus, image grainy or roughness of image surface significantly appears to create an excessive feeling of gloss, causing a problem of deterioration in quality of printed characters and images, compared with a case of back-printing in which process-color ink is jetted and then background-color ink such as white ink is jetted.
An inkjet recording apparatus has not yet been developed which can record characters, symbols, pictures, and the like in process colors with a background in a special color such as white, and can create a uniform feeling of gloss with transparent ink on a recording medium having no image receiving layer.